Rio de Janeiro passed its first 2016 Olympics test with the end of an international sailing regatta on Guanabara Bay, one of the most heavily criticised venues in a city under attack for disorganisation and construction delays.

During the week-long event many athletes and coaches were surprised to find the bay's notoriously dirty water - infamous for raw sewage, floating garbage, boat-battering debris and animal corpses - to be far cleaner than expected.

"I noticed a big difference, there was a lot less rubbish in the water than there was a year ago," said Jo Aleh, helmswoman for the winning New Zealand team in the Women's 470 class at the end of that competition last weekend.

A long-time critic of the venue, Aleh said she preferred the pristine waters of her Pacific Island nation and still felt Guanabara Bay needed work. But she pointed out that many races are being held outside the bay in the open ocean where the water is "just fine."

Nor, she and other sailors added, is Rio the only important world sailing venue with water-quality issues. Pollution-fed algae blooms threatened Olympic sailing at the Beijing Games in Quingdao in 2008 and were only resolved in the weeks before the event started.

Concern Rio won't meet its cleanup goals has eased enough that Jorge Fundak, sports director for the Austrian sailing federation, is pushing organisers to hold more races in the bay and fewer in the cleaner open ocean. It is harder for TV to cover events outside the bay, he said.

Eilidh McIntyre, part of the two-person team that took ninth place in the Women's 470 Class, said concerns about the water were understandable but exaggerated.

"Most days the water was crystal clear," McIntyre said. "We saw dolphins. We did see a dead dog. But you can run into something like that almost anywhere."

State officials responsible for water quality acknowledge the Guanabara Bay still has serious pollution problems, but say they are concentrated away from the sailing venue.

Alistair Fox, head of competitions for the International Sailing Federation (ISAF), said his organisation supervised Rio's water-testing program and was satisfied that the water in the race courses is meeting local and international standards.

Problems still surface after major rainstorms when sewage from illegal connections or shantytowns gets mixed with rainwater and flushed into the bay, said Jose Candido Muricy, a Rio government spokesman for Olympic and environmental issues.

One of the outfall points is in the yacht harbor, the Marina da Gloria, where the sailing events will be based.

Tests showed the water quality to be good during the test regatta probably because it was held, just as the Olympics will be, during the low-rain Southern Hemisphere winter, he said.

But Muricy promises to eliminate that threat. An interceptor sewer should be completed by September 2015 to channel sewage away from the Marina and the bay.

Its construction was delayed by the collapse of Brazilian tycoon Eike Batista's EBX Group, which had promised to pay for half the project, he said.

As for the debris, Rio has closed old landfills that sloughed garbage into local rivers and is completing a network of "eco-barriers" on major bay tributaries to trap garbage before it can pollute the bay.

A fleet of garbage-collection boats also regularly works on the bay to scoop up stuff the barriers miss, he said.